Compensation for vertical component of earth&#39;s magnetic field by color triad displacement



J. L. HUDSON 3,187,650 TIC 'S MAGNETIC June 8, 1965 COMPENSATION FOR VERAL COMPONENT OF EARTH FIELD BY COLOR TRIAD DISPLACEMENT Filed Dec. 50,1955 7'0 CWT I005 user/m1. compommr or m: 3/ EARTHJS 'MAg/vEr/c HEL o.

x. .lln d 5% w z INVENTOR. Hwso/v Joy/V l employed for the purpose ofUnited States Patent Ofitice 3,187,650 COMPENSATION FOR VERTICALCOMPONENT OF EARTHS MAGNETIC FHELD BY CULGR TRIAD DISPLACEMENT John L.Hudsen, Lancaster, Pa., assignor to Radio Corporation of America, acorporation of Delaware Filed Dec. 30, 1955, Ser. No. 556,649 1 Claim.(Cl. 95-1) This invention relates to scopes and other cathode-ray tubesof the kind containing an apertured mask through which beam-electronspass in their transit to preselected elemental areas on the mosaictarget surface of a nearby screen, and to improvements in the art ofmaking such tubes.

In the operation of cathode-ray tubes of the subject (masked-target)variety the accuracy with which the beam-electrons strike the individualelemental areas of the mosaic-screen may be adversely affected by thepresence of the earths magnetic field in the paths of the electrons.This fact has long been recognized and it is now more or less standardpractice to compensate for the beam-deflecting effect of the earthsfield by providing such tubes with a magnet-assembly or a purity coilcapable of directing the electron-beams to paths which terminate,ideally, at the centers of the individual electron-sensitive dots (orlines) which make up the mosaic target-surface of the screen. Suchauxiliary devices operate quite satisfactorily to perform their intendedfunctions but their use is subject to certain disadvantages, Ininstances where the auxiliary magnetic field must be made of arelatively large magnitude, to effect resolution of the resulting image,picture or raster. Picture quality may be further adversely affected ifthe purity field interacts with the field of some other internal orexternal component of the set.

The present invention provides a cathode-ray tube of the masked-targetvariety wherein picture defects casioned by the presence of the earthsmagnetic field in the path of the beam-electrons are minimized by theprovision of a target-assembly of a construction which permits the useof a correcting field of a magnitude considerably less than that of theauxiliary fields heretofore maintaining said electrons in their intendedpaths.

The principle upon which the invention is based will the more readily beunderstood when it is appreciated that the vertical component of theearths magnetic field, at any particular geographic location, has aconstant eliect upon an electron-beam regardless of the orientation ofthe tube in which the beam is contained, provided only that the axis ofthe tube is substantially horizontal. (The effect of the horizontalcomponent of the earths field upon an electron beam tube varies, at anygiven geographic location, with the orientation of the tube.)

The fact that the vertical component of the earths field has a constanteffect upon an electron-beam permits a constant correction factor(heretofore supplied entirely by an auxiliary magnetic device) to bebuilt in the tube. Stated generally, this is achieved bythe provision ofa tube during the screen-plotting operation and are subsequently mountedWithin the tube in a position whereat In making a screen-assernbly, inaccordance with the 3,187,650 Patented June 8, 1965 invention, therelative location of the screen pattern, and of the pattern of aperturesin the mask, with re spect to the location of the electron-gun or guns,is fixed during manufacture of the tube preferably by olfsetting thescreen-assembly (i.e. the screen and its mask) with respect to the lightsource employed in plotting the location of the elemental areas of whichthe mosaic target surface of the screen is comprised. When a particulartube is to be employed at a particular geographic location the exactdegree of off-set employed in the screen-plotting operation can bedictated by the magnitude of the vertical component of the earthsmagnetic field at that particular geographic location. In this case, noauxiliary magnetic field whatsoever is required to commagnetic device 18ordinarily so small that it has pracor raster and, in any event, is lessthan is required for tubes made in accordance with the prior art.

The invention is described in greater detail in connection with theaccompanying single sheet of drawings, wherein:

FIG. 1 is a partly diagrammatic longitudinal sectional view, as viewedfrom above, of a 3-gun tri-c0lor kinescope of the shadow-mask dot-screenvariety; the drawing being marked with a line (yy) showing that thecenter of the mosiac pattern of the screen is off-set lateral- 1y, fromthe central axis (xx) of the tube, as dictated by the present invention;

FIG. 2 is an enlarged fragmentary rear-elevational view of the screenunit of the color-kinescope of FIG. 1;

FIG. 3 is a greatly enlarged front elevational view of the centralportion of a color dot-screen similar to the one shown in FIG. 2 butwith the drawing marked to illustrate a lack of register between thescreen-dots and electron-spots occasioned, in a conventional tube, bythe effect of the vertical component field upon the path of theelectrons;

FIG. 4 is a partly diagrammatic sectional view of a photographiclighthouse showing the apertured mask and screen-plate of acolor-kinescope set-up thereon in an off-axis position with respect tothe light-source position,

invention. One type of cathode-ray tube to which the invention isapplicable is a 3-gun tri-color kinescope of the shadow mask variety. Asshown in mask variety.

The mosaic screen 17 (see FIG. 2) comprises a multiplicity (usually300,000 or more) of triads (i.e. groupsof-three) 'of red (R), blue (B)and green (G) colorphosphor dots. The phosphor-dots are here arranged ina hexagonal pattern; that is to say each dot is surrounded by six otherdots, alternate ones of said other dots being of a second color-responsecharacteristic and the intermediate ones of said other dots exhibiting athird colorresponse characteristic. An electron-transparent, lightreflecting, metallic (e.g. aluminum) film 17 (FIG. 1)

(eg 18 kv.) to provide a V g (green) are here shown in field designed toinfluence the target variety the relationship or phor areas.

3, covers the entire target surface of this dot-like mosaicscreen, andforms an electrical connection to the cap 13 of the envelope ll.

The other element or shadow mask of the bi-part screen-unit 17, 19comprises a suitably curved thin-metal plate 19 containing amultiplicity of apertures 1% arranged in the same systemmatic(hexagonal) pattern as the phosphor screen-dots; there being one maskaperture for each triad (REG) of dots. As indicated by the singleelectrical connection 21 (FIG. 1) the metallized screen 1'7 and the mask19 are here operated at the same potential field-free spacetherebetween, as is usual in color-kinescopes operating on theshadowmask principle. The mask has an integral rim 1% and is supportedabout said rim on three or more pegs 23 which project radially inwardfrom the inner surface of the cap portion 13 of the envelope. Theconnection between the pegs 23 and the rim 19b is such as to permit themask to be removed from the cap during the three (later described)emulsion-coating and developing operations incident to laying down thethree color-phosphors (RBG) on the glass screen-plate 15.

The tubular glass neck 3 of the envelope 1 contains a battery of threeelectron-guns ZSr, 25b and 25g each of which is allotted to a particularscreen-color. The guns are here shown arranged delta fashion about thehori- 'zontal axis x-x of the envelope (as in Schroeder USP 2,595,548)so that their beams converge near a surface of the mask 19, where theirpaths may cross and proceed to the different color-phosphor dots.Alternatively, a single beam may be employed, in which latter caseauxiliary means may be provided as, for example, in Jenny 2,611,099, forsequentially shifting the single beam to positions corresponding to thatof the several beams in a multi-gun tube.

Referring still to FIG. 1. Here, as in the Schroeder patent, therequired horizontal and vertical scanning movements are applied to allthree of the electron beams from the guns 25 by a common deflecting yoke27 which will be understood to comprise two sets of electromagneticcoils (indicated by the double current leads) disposed at right anglesto each other on the glass neck 3. The colorcenters r, b, g of the threebeams r (red) b (blue) and the normal plane-of-deflection AA, i.e.approximately at the center of the yoke 27.

As in Friend USP 2,719,249 an auxiliary magnetic paths of the threebeams is provided by a purity coi 29 mounted on the neck 3 surroundingthe horizontal axis xx of the tube. and adjustably energized by a sourceof current, exemplified by the battery 31 and rheostat 33.

The color-tube of FIG. 1, more or less standard construction. However,it will be observed that the drawing is marked with a horizontal line oraxis y-y which, as will hereinafter more fully appear, passes throughthe center of the mosaic pattern of phosphor dots on the screen, axisx-x, about which the electron guns 25 are grouped, a distance calculatedto compensate for the lateral deflection forces to which theelectron-beams are subjected by the vertical component of the earthsmagnetic field. the amount of, and the direction in which, the mosaicpattern of the screen is off-set from the horizontal axis xx of the tubeis set forth in the following description of the manner in which thescreen-unit is manufactured.

In the manufacture of cathode ray tubes of the masked phosphor dots (orlines) and the mask-apertures is determined, usually, by light raysemanating from a small source located, successively, at pointscorresponding to the color-centers of the completed tube. When the tubeis I operated, electron-beams take the place of the light-rays that wereused in plotting the location of the color phos- Hence one might assumethat each beam would land squarely upon the particular dot toward whichregister" of the coloras thus far described, is of and is off-set fromthe V geographic location at which the it is directed. Such, however, isnot the case because the electron beams, unlike the light-rays, aresubject to lateral deflection by the presence of the earths magneticfield. Thus, referring now to FIG. 3, the effect of the verticalcomponent of the earths magnetic field, in the absence of compensation,is to deflect the beams r, b and g an appreciable distance to the left,as viewed by an observer looking at the screen from the front of thetube. The amount or degree of misregister produced by the verticalcomponent of the earths magnetic field is the vector summation of themisregister produced by said field in the gun region plus themisregister produced by said field in the space between theplane-of-defiection (AA) and the screen. These two misregistercomponents are oppositely directed and the actual misregister observedin an uncorrected tube is a dilference error, in the direction of thelarger component. At any given location the actual distance that thebeams are moved oif the centers of the elemental screen-areas (RBG) bythe vetrical component of the earths magnetic field is about twice asgreat in an unshielded glass tube than it is in a metal tube, or in onehaving a metal shield about its cone. was observed that at Lancaster,Pennsylvania (where the vertical component of the earths magnetic fieldis approximately 0.53 gauss), the amount of misregister at the centerposition of the screen of an unshielded 22 rectangular glass tri-colorkinescope is approximately 8 mils whereas in a 21" tri-color tube havinga metal envelope it is approximately 4 mils.

' The general formula for determining the distance (AS) that the top capand light-source must be displaced with respect to each other in orderto place the phosphor dots at the terminals of the three electron-beamswhen the latter are'subjected to the lateral deflecting force of thevertical component of the earths magnetic-field, is:

' ured along the axis of the tube.

Assuming now that a mosaic screen-pattern has been plotted or laid downin accordance with'the above formula, then the distance that saidpattern should be off-set from the horizontal axis (x-x) of the finishedtube is expressed by the formula:

AE= AS As a practical matter, the objects of the invention are achievedin an unshielded tube when AB is from 0.003 to 0.020 of an inch. In ashielded tube (eg one having a metal cone) AE may be of the order offrom 0.0015 to 0.010 of an inch. As previously pointed out the directionof the off-set, north of the earths magnetic equator, is to the left ofan observer looking directly at the obverse surface of the screen, andsouth of said equator the oif-setis in the opposite direction, i.e. tothe right.

Where mass-production is involved it is not convenient to change thepositionof the screen-pattern for each tubes'are to be used. Nor is itnecessary to do so since the objects of the invention are fulfilled bythe provision of a degree of off-set which permits a reduction in themagnitude of the magnetic field supplied by the, purity coil (or similarmagnetic device) to a value which will not noticeably affect theresolution of the electron-images which the tubes are designed tohandle. Accordingly, in applying Thus, it

(since AS=.00425 28.2)

the shift of the screen-assembly with respect to the axis (x x) of thetube, required to effect the desired comat a particular color-center, inthe tubes plane-ofdeflection, or (b) by leaving it in its said usualposition and placing the front-end or top-cap (13) of the tube therequired distance off the central axis x-x of the to be mounted in thefinished tube. The light-source 37, instead of being mounted forrotation about the cenmosaic pattern impressed thereon by reason of thepresence of the apertured mask 19 in the path of the light rays from thepoint source 37. Assuming that the point 37, in the plane-of-deflectionA-A, is the one to be traversed by the red beam of the finishedkinescope then the emulsion coating 39 on the inner surface of the glassplate 15 should contain a red-phosphor such, for example as manganeseactivated zinc phosphate. Since, as previously mentioned, the aperturedshadow mask 19 is removably supported on the inner surface of the cap 13by three or more pins 23, it may be removed from the assembly during thethree emulsion-coating and de veloping operations incident to layingdown the three (red, blue and green) color-phosphors. When the tube cap13 is restored pedestal 35 for the blue and the green exposures, thelight-source must of course be moved about the new axis zz from exposureto bring green guns,

When the screen p 13 is sealed resp to its indicated axial position onthe its position during the previous it to the position of the blue andectively.

source in the previously described off-axis position the mosaic-patternof the screen will be vertical component of the olf-set from the for thedeflecting eifect of the earths magnetic field upon the beam-electronsemployed in scanning the screen.

What is claimed is:

The method of compensating for misregister due to the beam-deflect ingeffect of the vertical component of the earths magnetic field upon thebeam-electrons in set from a colordefined by Where Pis the said apertureq is the distance between mosaic variety, ing photographically elementalscreensaid method comprislaying-down a mosaic pattern of areas upon saidtarget surface with the in said plane-of-deflection at a point oifcenter in said plane a distance (AS) the formula:

mponent of the earths magnetic field,

distance between said plane-of-deflection and d mask and said mask andscreen as measured along said horizontal axis, and thereafter mou tubewith dictated by the foregoing formula.

References Cited by the Examiner UNITED STATES PATENTS ROBERT SEGAL,Acting Primary Examiner. RALPH G. NILSON, GEORGE N. WESTBY,

Examiners.

